Most modern naval and merchant ships have in-line propulsion systems utilizing at least one inboard engine that drives an external propeller in the form of a shaft.
Depending on the size of the vessel and required performance, the power requirements of the engine(s), the size and number of propellers and the consequent size and number of the propeller shafts can be calculated. To transmit the power from the engine to the propeller, each shaft needs to be supported in a bearing system. The aft internal structure supporting the shaft with bearings is known as the stern tube. The propeller shaft may additionally be supported by brackets external to the hull, to which additional shaft support bearing systems are fitted. For this application, the shafts generally have a minimum diameter of 150 mm with a resultant minimum wall thickness for the bearing of 15 mm.
Traditionally, the shaft support bearing has been based upon a white-metal/oil lubrication system, although non-metallic/water lubrication systems are also utilised, primarily for naval applications.
In oil lubricated systems, there is a sealing system for the stern tube. The seal has dual functions: to prevent the loss of oil lubricant required by the shaft/bearing, and to prevent the ingress of water into the stern tube.
It is generally accepted that oil seals are prone to damage from extraneous matter found in the aqueous environment. The damage may in certain circumstances cause oil loss with resultant damage to the bearing, or water ingress, which may also be detrimental to the bearing.
For white-metal bearings, the loss of oil may result in an excessive temperature rise, which may cause irreparable damage to the shaft and bearing. Similarly the ingress of water may have a detrimental effect upon the lubrication system and may also result in irreparable damage to the shaft and bearing.
An alternative to the white-metal/oil lubrication system is a non-metallic bearing/oil lubrication system. A typical example of the non-metallic bearing material used in this type of application is the asbestos fibre reinforced phenolic resin bearing material AWA8OH@ manufactured by Railko Ltd. The non-metallic, asbestos fibre reinforced phenolic resin bearings have an increased tolerance to loss of oil lubricant and water ingress.
Non-metallic bearings have traditionally been manufactured to established designs derived for white-metal bearings, and can thus be substituted for the metallic bearings. However, during the course of design, non-metallic bearings have an additional step in the calculation of operational clearance between shaft and bearing. This additional calculation is necessary to accommodate any swelling characteristics in either oil or water of the non-metallic material, and results in an increased running clearance.
The degree of swelling of the non-metallic bearing material is dependent on the radial thickness of the bearing, but can be as much as 1.5%. As a result, it may be necessary to provide a running clearance of 1-3 mm. A large running clearance will allow excessive radial displacement of the shaft and may cause increased friction and heating of the bearing.
A shaft bearing system that not only minimizes damage from extraneous matter in aquatic environments, but also the relatively large running clearance needed when using certain non-metallic bearing elements, would therefore be advantageous. The present invention provides such a shaft bearing system.